RF devices conventionally employ couplers for sampling an RF signal combining signals asymmetrically for line monitoring, power measurements, and load source isolators. Couplers comprise three basic types of devices: unidirectional, bidirectional, and dual directional.
Unidirectional RF couplers are four-port devices with a main line having an input and an output, and a coupled line with a terminated input and a coupled output. The power passing to the output on the main line is combined with the coupled output, but the coupled output is isolated from the main line output. Any reflected power from the main line output is coupled to the termination.
Bidirectional RF couplers are similar to unidirectional couplers, but do not provide termination on the fourth port (coupled input). Main line power is coupled to the forward output of secondary line and reflected power is coupled to the reflected output. For isolation to be achieved, coupled outputs must be obtained through well-matched terminations at each port.
Dual directional RF couplers are four-port devices that have two unidirectional couplers. They can be connected back-to-back in series, with the main line output of the forward coupler connected to the output of the second coupler; or integrated into one device with a single main line and two secondary lines. Integration in a single device provides several advantages, including a shorter or more compact unit. In addition, since there is only one main line, insertion loss is reduced and high isolation is more easily attained.
Selecting RF couplers requires an analysis of performance specifications. Continuous wave (CW) average power measures one-way transmissions through the main line under matched load conditions. Coupling is the ratio of the power measured at the output port relative to that measured at the forward coupled port. Directivity is the ratio of the power measured at the reverse coupled port relative to that measured at the forward coupled port when the output port is ideally terminated. Insertion loss is the total RF power transmission loss resulting from the insertion of a device in a transmission line. It is defined as the ratio of signal power at the output of the inserted device to the signal power at the input of the inserted device. Coupling, directivity, and insertion loss are all measured in decibels (dB). Voltage standing wave ratio (VSWR) is a unitless ratio ranging from 1 to infinity that expresses the amount of reflected energy at the input of the device. A value of 1 indicates that all of the energy passes through. Any other value indicates that a portion of the energy is reflected. Other performance specifications for RF couplers include frequency range, return loss, and reflected power.
These known couplers, including for example adjacent strip lines, require well controlled gaps that require complex manufacturing techniques. Known mult-port couplers include multiple strip lines spaced around a ring. Contamination between the gaps may contribute to poor efficiency of the coupling.
Accordingly, it is desirable to provide a multi-port coupler for use in an RF device that does not require controlled gaps between adjacent conductors. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.